1. Field of the Invention:
The present invention generally relates to living tissue stimulators and, more particularly, to new improved implantable living tissue stimulators.
2. Description of the Prior Art:
Various devices are presently in existence which are implantable in a living body in order to stimulate selected body tissues. Among the best known living tissue stimulators is the cardiac pacemaker, which provides stimulating pulses to a patient's heart via one or more electrodes connected to the pacemaker through electrode leads. Some commercially available implantable pacemakers consist of pulse generating circuitry and a proper power source which are encapsulated in resinous material, e.g., epoxy, which is intended to serve as electrical insulation and as a body fluid impervious barrier. As is appreciated, the body fluid is a saline solution which is electrically conductive.
From experience with such pacemakers it has been discovered that even though the encapsulating material is intended to serve as an impervious barrier to the body fluid it is not a very good barrier. The body fluid tends to impregnate the encapsulating material, particularly through small cracks therein. Such impregnation presents a significant danger to the patient. The danger is due to the fact that currents from the power source and/or from the pulse generating circuitry may flow out of the pacemaker through the impregnating body fluid to the rest of the body. Such currents may damage body tissue, thereby endangering the patient. The danger of current flow from the pacemaker to the rest of the body is particularly significant in a pacemaker whose power source is rechargeable by means of an external magnetic field, due to the presence of significant voltage and currents during recharge.
In addition, it is highly desirable to protect the pacemaker circuitry from any currents from external sources which may flow to the circuitry through the impregnating body fluid, thereby potentially affecting the proper operation of the pacemaker circuitry. Thus, a need exists for means for use with an implantable pacemaker to inhibit the flow of current through the body fluid (except to the electrodes through their leads) either out of the pacemaker to the rest of the body or into the pacemaker from an external source.
Also, in any implantable pacemaker it is highly desirable to protect the stimulator circuit components from adverse effects due to an external alternating magnetic field, which may interfere with the proper operation of the stimulator circuitry as well as heat up its components. Such protection is of particular importance in an implantable rechargeable pacemaker which includes a pickup coil in which current is induced by an external alternating magnetic field to recharge a power source. e.g., a battery. At present an implantable rechargeable pacemaker is available in which all the circuit components are hermetically sealed in a hermetic metal container. In such a pacemaker, a significant portion of the power induced by the external alternating magnetic field is dissipated as heat in the metal container and metal circuit components. The portion of the total induced power which is converted into useful battery charging power is small. Thus, a need exists for an improved implantable rechargeable hermetically sealed pacemaker.